This invention relates to polymer emulsions, more particularly to a novel process for preparing stable aqueous polymer emulsions of low viscosities having higher solids contents than were heretofore obtained by using conventional emulsion polymerization techniques.
Aqueous emulsions of styrene-butadiene type copolymers have been known for many years. Generally, the procedures utilized to prepare such emulsions involve the addition with rapid stirring of one or more of the monomers to water simultaneously with, or subsequent to, the addition of surfactants, emulsifying agents, polymerization catalysts or initiators, and, in many instances, a protective colloid-forming substance. The mixture is held at polymerization temperature until complete polymerization of the monomers has occurred. The resulting copolymer emulsion, upon cooling and filtering, can be used in many applications such as paints, coatings, adhesives, binders, caulking compositions and the like.
U.S. Pat. No. 3,637,563 has disclosed a novel process for the preparation of aqueous emulsions, wherein it is possible to achieve, in most instances, polymer solids contents in excess of about 50 percent, by weight, with attendant low viscosities and shelf stability.
However, the process disclosed in this invention has not been entirely successful in achieving extremely high solids, low viscosity aqueous emulsions utilizing styrene and butadiene as the predominant copolymerizing monomers. Furthermore, at solids contents of near 75 weight percent, it has been especially difficult to prepare low viscosity, shelf-stable polymers according to the process disclosed.
Thus, it is an object of the present invention to provide a process for preparing predominantly styrene-butadiene copolymer emulsions.
It is also an object of this invention to provide a process for preparing styrene-butadiene copolymer emulsions having extremely high solid contents, but handleable viscosities. These and other objectives are obtained by preparing polymers according to the process disclosed hereinafter.